So far, an illuminating device has been, in general, of such a type that performs radiation of light rays concentratedly in the visible part of the light rays. And, in the image pick-up device using a television camera, the image pick-up operation is done under illumination of light radiating from the visible part of the light rays.
However, in the image pick-up method using the light rays in the visible part, there take place various inconveniences owing to an object and the image pick-up device being seen during the image pick-up operation.
For example, in various crime-preventing monitoring devices and visitor congnition devices installed in shops, banks, factories, residences, and so on, there has been adopted a method, in which the image pick-up operation is done by capturing visible light rays from an object (for image pick-up) which has been irradiated with luminous light in the visible part of light by means of a video-camera, and displaying video signals thereof on a Braun tube of a television set, or inputting the video signals in a video tape for later reproduction of the picked-up image.
Accordingly, in the crime-preventing monitoring device, an intruder is able to readily recognize existence of the monitoring device, which is disadvantageous. Also, in the visitor congnition device installed at the main gate or rear gate of a house, there are such disadvantages that the visitor is dazzled by the illuminating light, or feels displeasure at his knowledge of being pictured by a camera.
Thus, in the conventional image pick-up device which picks up images under visible light, the above-described disadvantages were unavoidable.
Also, as described in "The Journal of Japan Society of Illumination Technology", No. 43, Vol. 1, pp 21 to 28, there has been known a noctovision, etc., which projects near-infrared rays onto an object, then amplifies light reflected from the object, and observes the object with naked eyes through an image tube. This, however, is not an image pick-up device, but is used for particular purposes, which is therefore not a general purpose device.
On the other hand, as a light source for emitting light in the near-infrared part of the light, there has been one, in which an incandescent electric bulb is covered with an appropriate filter. However, owing to inferiority in illuminating efficiency of the incandescent electric bulb and the absorbing effect of the filter, the light emitting efficiency in the near-infrared region was so poor as to constitute a problem. On account of this, attempt has been made as to obtaining near-infrared light emission with a fluoroscent lamp which has higher light emitting efficiency than the incandescent electric bulb. For instance, as described in Japanese Examined Patent Publication No. 42436/1976 and Journal of IES, April (1974), pp 234 to 236, there can be obtained a near-infrared light emitting fluorescent lamp having its light emitting wavelength region of from 650 to 900 nm, with its peak being in the vicinity of 740 nm, by use of iron-added lithium aluminate fluorescent material. While, however, this fluorescent lamp is excellent in its higher light emitting efficiency in the near-infrared region than that of the incandescent electric bulb, it has such disadvantage that the fluorescent material thereof becomes deteriorated with lighting and its light output lowers abruptly.